Why gunpowder contaminants give false result with DPA test reagent?

The DPA (diphenylamine) reagent test is often used to detect the presence of nitrate or nitrite residues on surfaces but gunpowder itself contains nitrate therefore giving false results due to the gunpowder itself and not only gunshot residue. As a consequence of this reaction a blue purple compound forms that might indicate presence of gunshot residue, but only on the condition that no gunpowder contaminants were encountered.